The functional importance of the Cspg2/versican gene to early heart formation has been clearly demonstrated by the Cspg2 null mice called heart defect (hdf). Total absence of the Cspg2 gene literally means the end of heart development at a stage prior to cushion formation and outlet segment growth. This creates a problem for determining the functional mechanism and significance of Cspg2/versican at later stages of heart development involving septa and valve maturation when there is the highest relevance to the cardiac defects most often seen in live births. To approach these questions, we have begun to analyze to a new mouse model that is a mRNA splice form null of Cspg2. Our analysis shows that mice homozygous for the deletion can survive and breed, but also possess cardiac defects highly relevant to at least 1/3 of all live birth defects. Our central working hypothesis is that Cspg2/versican modulates normal EGFR signaling in the heart cushions and regulates cell-matrix signaling needed for growth and muscularization of the forming heart cushions. The major questions to be addressed are 1) how does versican play a role in modulating EGF signaling and downstream targets needed during cushion mesenchyme formation, growth and maturation;ii) what is the active or permissive role of versican in the two fundamental steps of outlet cushion myocardialization;iii) what is the role of versican in stabilizing the myocardial phenotype in maturing cushion mesenchyme? The purpose of the proposed studies are to determine versican's function at later stages that impact directly on live birth heart defects. We will use a combination of in vitro bioassays, morphology, high throughput 3D confocal imaging and high throughput proteomics to determine versican's function. The Specific Aims are: 1) Determine the cellular and molecular mechanisms through which versican splice forms modulate formation and differentiation of the cushion mesenchyme.;2) Determine the mechanism through which the versican V2/V0 splice form mediates the process of myocardialization that is associated with alignment of the cardiac outlet septa to the ventricular outlets.;3) Determine how the defective phenotype in the V2/V0 null hearts is modulated when placed on genetic backgrounds affecting EGFR signaling and heart development. Several morphogenetic mechanisms involving cushion mesenchyme have been reported to be necessary for remodeling the U-shaped heart tube into four chambers. However, the active or permissive role of the Cspg2 gene product, versican, on these central mechanisms is recognized but largely unexplored. The V2/V0 null mouse provides the first mammalian model in which we can directly investigate the disruption of these central mechanisms of cardiac outlet integration that result from a partial absence of the extracellular matrix proteoglycan versican.Although several morphogenetic mechanisms involving cushion mesenchyme have been reported to be necessary for remodeling the U-shaped heart tube into four chambers, the active or permissive role of the Cspg2 gene product, versican, is recognized but largely unexplored. We have shown in the heart defect mouse (Cspg2 null), the failure to form cushions results in early embryonic lethality. The V2/V0 null mouse in this proposal provides the first mammalian model in which we can directly investigate the role of versican in the heart at developmental stages that directly impact clinically relevant live birth heart defects.